A modern well designed and operated secondary waste treatment works, at times, is capable of the discharge of large quantities of suspended solids into the receiving waters. The effect of these suspended solids upon the receiving waters is well-known, and has resulted in various Regulatory Agencies establishing maximum limits of suspended solids that may be legally discharged into certain waters.
The need to control the discharge of suspended solids, and insure water quality is most effectively met by filtration of the treated waste prior to discharge to receiving waters.
Initially, the filters that are utilized primarily for potable water filtration were adapted to the direct filtration of treated waste water with limited success. These types of filters are extremely limited in solids capture capacity, and have little or no tolerance for solids shock loadings.
A very effective waste water filter has been developed and is described in my U.S. Pat. No 28,458, reissued July 1, 1975. In that patent a method of increasing the filter run of a particulate media filter is materially improved by creating currents sweeping over the media surface. The patent is incorporated by reference herein as the general art to which the present invention is directed.
A further improvement to the filter described in my U.S. Pat. No. 28,458 is disclosed and claimed in my U.S. Pat. No. 3,817,378 issued June 18, 1974. In this patent, which is also incorporated as reference, there is disclosed a waste water filter wherein air is forced upwardly through the filter bed between the backwash cycles thereby regenerating the filter media surface, and increasing the length of the filter run between backwashes.
Treated unfiltered waste water may contain substantial quantities of grease and oil in addition to varying quantities of suspended solids.
Researchers in the waste water treatment field claim that the single most abundant ingredient in domestic sewage may be grease. The classes of compounds generally found in the sewage grease fraction include hydrocarbons, glycerides, sterols, fatty acids and compound lipids. Because most of these compounds are insoluble in water, they exist on an emulsion uniformily dispersed in water, or as a separate layer which in part may have coalesced into grease balls.
A portion of grease present in waste water is in the form of a colloid or a supracolloid. This colloidal and supracolloidal fraction of grease may represent as much as one-fourth to one-half of the total grease fraction present in the waste water. This grease emulsion may be further strengthened by large quantities of detergent or other emulsifying agents present in the waste water.
Quantities of grease varying at times to substantial amounts may then be discharged from sewage treatment plants, along with varying quantities of solids, as a function of design, loading and operation of these treatment works.
Waste treatment plant effluent containing significant amounts of colloidal or coalesced grease add a new and difficult dimension to the waste water filtration process.
The grease fraction will penetrate into the filter media, the interfacial film between the colloidal droplets and the treatment plant effluent is physically broken, due in part, to the tortuous path formed by the media grains. Once the interfacial film is broken the grease droplets will coalesce on the grain surface. The speed with which this interfacial film is broken is, in part, a function of strength of the emulsifier present in the waste water. This adhesion of grease to the media will form in a tenacious film around the media grain.
This tenacious grease film will cause the very fine media to adhere together forming clumps that block passage of air or water. This grease film will also cause media grains to attract and hold suspended solids to the grain surface, the film acting as a binder with other grains to block flows of air and water in all directions.
This packing of filtering media will quickly reduce the efficiency of the filter, and in turn will cause air and water rising in any cleaning cycle to flow around the tightly packed media volumes, reducing or negating the effectiveness of the cleaning cycle.
An improvement in filter underdrain structure is described and claimed in my U.S. Pat. No. 3,840,117 dated Oct. 8, 1974, wherein air is distributed upwardly in an even manner over the total area of the media bed to periodically remove plugged or packed areas of the bed. The underdrain also eliminates the possibility of high velocity channelling around densely packed media through uneven distribution of air or backwash liquid.
Commonly used filter underdrain structures are designed to develop even distribution of backwashing water throughout the filter area, generally at wash rates that may vary from 15 to 30 gallons per minute. These backwash rates represent a rising velocity of 24 to 48- per minute or velocities much less than 1" per second. This low energy wash velocity is easily diverted around the grease packed areas within the media bed.
A further improvement is also described and claimed in my U.S. Pat. No. 3,840,117 wherein the backwashing liquid flows as a series of separate high velocity liquid jets passing upwardly in a generally vertical direction, acting as augers boring upwardly, the jets being closely spaced providing a uniform cleaning action of the media.
The various grease clogged volumes within the filtering media remain generally intact following backwashing by the conventional means described irrespective of the backwash rates. These filters will quickly become clogged again. These short filter runs can be lengthened by increasing the effective size of the media, or chemical cleaning the media. Increasing the media size will sacrifice filtrate quality. Chemical cleaning the media is difficult.
The grease build up in filter media has been at times identified as slime, and operators and treatment plant designers have recommended pre-chlorination of the treated waste in an attempt to retard "slime" growth. In effect, the operator is attempting to strengthen the emulsion, and thereby allowing some grease to pass through the media. This technique is of course counter productive, and has limited success, primarily with coarse media. Pre-chlorination may cause deeper penetration of grease in very fine media.
Increasing filter media size will permit longer runs at reduced quality, but the media grains will still coat with grease causing high velocity channels during the backwash with some media loss, the filter, however, will eventually still have to be chemically cleaned.
The underdrain improvement described in my U.S. Pat. No. 3,840,117 backwashes fine filter media without the severe problems associated with conventional underdrains. However, after a period of time a very fine grease film will develop on the media grains. Eventually, the media must be chemically cleaned.
Filter cells are cleaned conventionally, by closing the filter drain, adding a chemical cleaning solution to the filter inlet, and/or sprinkling some cleaning solution on to the media. The cleaning solution must be worked into the media, generally by hand to insure the complete immersion of each media grain in the solution. The fact that personnel must enter the filter cell may be dangerous, yet required, and especially so if sodium hypochlorite is used as the cleaning solution. Eventually, the media may also have to be replaced.
All of these disadvantages are overcome by the present invention which is directed toward a waste effluent filter of the type preferably utilizing a particulate filter bed and an underdrain structure as described in my U.S. Pat. No. 3,840,117. The present invention is also directed towards waste effluent filters utilizing conventional even flow type underdrain systems.